The demand for energy and the hydrocarbons from which that energy is derived is continually rising. The hydrocarbon raw materials used to provide this energy, however, contain difficult to remove sulfur and metals that hinder their usage. Sulfur can cause air pollution, and can poison catalysts designed to remove hydrocarbons and nitrogen oxide from motor vehicle exhaust. Similarly, other metals contained in the hydrocarbon stream can poison catalysts typically utilized for removal of sulfur through standard and improved hydro-desulfurization processes whereby hydrogen reacts under extreme conditions to break down the sulfur bearing organo-sulfur molecules.
Extensive reserves of shale oil exist in the U.S. that will increasingly play a role in meeting U.S. energy needs. Over 1 trillion barrels reserves lay in a relatively small area known as the Green River Formation located in Colorado, Utah, and Wyoming. As the price of crude oil rises, the resource becomes more attractive but technical issues remain to be solved. A key issue is addressing the relatively high level of nitrogen contained in the shale oil chemistry after retorting as well as addressing sulfur and metals content.
Shale oil characteristically is high in nitrogen, sulfur, and heavy metals which makes subsequent hydrotreating difficult. According to America's Strategic Unconventional Fuels, Vol. III—Resource and Technology Profiles, p. 111-25, nitrogen is typically around 2% and sulfur around 1% along with some metals in shale oil. Heavy metals contained in shale oil pose a large problem to upgraders. Sulfur and nitrogen typically are removed through treating with hydrogen at elevated temperature and pressure over catalysts such as Co—Mo/Al2O3 or Ni—Mo/Al2O3. These catalysts are deactivated as the metals mask the catalysts.
Another example of a source of hydrocarbon fuel where the removal of sulfur poses a problem is in bitumen existing in ample quantities in Alberta, Canada and heavy oils such as in Venezuela. In order to remove sufficient sulfur from the bitumen for it to be useful as an energy resource, excessive hydrogen must be introduced under extreme conditions, which creates an inefficient and economically undesirable process.
Over the last several years, sodium has been recognized as being effective for the treatment of high-sulfur petroleum oil distillate, crude, heavy oil, bitumen, and shale oil. Sodium is capable of reacting with the oil and its contaminants to dramatically reduce the sulfur, nitrogen, and metal content through the formation of sodium sulfide compounds (sulfide, polysulfide and hydrosulfide). Examples of the processes can be seen in U.S. Pat. Nos. 3,785,965; 3,787,315; 3,788,978; 4,076,613; 5,695,632; 5,935,421; and 6,210,564.
An alkali metal such as sodium or lithium is reacted with the oil at about 350° C. and 300-2000 psi. For example 1-2 moles sodium and 1-1.5 moles hydrogen may be needed per mole sulfur according to the following initial reaction with the alkali metal:R—S—R′+2Na+H2→R—H+R′—H+Na2SR,R′,R″—N+3Na+1.5H2→R—H+R′—H+R″—H+Na3N
Where R, R′, R″ represent portions of organic molecules or organic rings.
The sodium sulfide and sodium nitride products of the foregoing reactions may be further reacted with hydrogen sulfide according to the following reactions:Na2S+H2S→2NaHS(liquid at 375° C.)Na3N+3H2S→3NaHS+NH3 
The nitrogen is removed in the form of ammonia which may be vented and recovered. The sulfur is removed in the form of an alkali hydrosulfide, NaHS, which is separated for further processing. The heavy metals and organic phase may be separated by gravimetric separation techniques. The above reactions are expressed using sodium but may be substituted with lithium.
Heavy metals contained in organometallic molecules such as complex porphyrins are reduced to the metallic state by the alkali metal. Once the heavy metals have been reduced, they can be separated from the oil because they no longer are chemically bonded to the organic structure. In addition, once the metals are removed from the porphyrin structure, the nitrogen heteroatoms in the structure are exposed for further denitrogenation.
The following is a non-limiting description of the foregoing process of using alkali metals to treat the petroleum organics. Liquid phase alkali metal is brought into contact with the organic molecules containing heteroatoms and metals in the presence of hydrogen. The free energy of reaction with sulfur and nitrogen and metals is stronger with alkali metals than with hydrogen so the reaction more readily occurs without full saturation of the organics with hydrogen. Hydrogen is needed in the reaction to fill in the where heteroatoms and metals are removed to prevent coking and polymerization, but alternatively, gases other than hydrogen may be used for preventing polymerization. Once the alkali metal compounds are formed and heavy metals are reduced to the metallic state, it is necessary to separate them. This is accomplished by a washing step, either with steam or with hydrogen sulfide to form a hydroxide phase if steam is utilized or a hydrosulfide phase if hydrogen sulfide is used. At the same time alkali nitride is presumed to react to form ammonia and more alkali hydroxide or hydrosulfide. A gravimetric separation such as centrifugation or filtering can separate the organic, upgraded oil, from the salt phase.
In conventional hydrotreating, instead of forming Na2S to desulfurize, or forming Na3N to denitrogenate, H2S and NH3 are formed respectively. The reaction to form hydrogen sulfide and ammonia is much less favorable thermodynamically than the formation of the sodium or lithium compounds so the parent molecules must be destabilized to a greater degree for the desulfurization of denitrogenation reaction to proceed. According to T. Kabe, A Ishihara, W. Qian, in Hydrodesulfurization and Hydrodenitrogenation, pp. 37, 110-112, Wiley-VCH, 1999, this destabilization occurs after the benzo rings are mostly saturated. To provide this saturation of the rings, more hydrogen is required for the desulfurization and denitrogenation reactions and more severe conditions are required to achieve the same levels of sulfur and nitrogen removal compared to removal with sodium or lithium. As mentioned above, desulfurizing or denitrogenating using hydrogen without sodium or lithium is further complicated with the masking of catalyst surfaces from precipitating heavy metals and coke. Since the sodium is in the liquid phase, it can more easily access the sulfur, nitrogen and metals where reaction is desirable.
Once the alkali metal sulfide has been separated from the oil, sulfur and metals are substantially removed, and nitrogen is moderately removed. Also, both viscosity and density are reduced (API gravity is increased). Bitumen or heavy oil would be considered synthetic crude oil (SCO) and can be shipped via pipeline for further refining. Similarly, shale oil will have been considerably upgraded after such processing. Subsequent refining will be easier since the troublesome metals have been removed.
Although the effectiveness of the use of alkali metals such as sodium in the removal of sulfur has been demonstrated, the process is not commercially practiced because a practical, cost-effective method to regenerate the alkali metal has not yet heretofore been proposed. Several researchers have proposed the regeneration of sodium using an electrolytic cell, which uses a sodium-ion-conductive beta-alumina membrane. Beta-alumina, however, is both expensive and fragile, and no significant metal production utilizes beta-alumina as a membrane separator. Further, the cell utilizes a sulfur anode, which results in high polarization of the cell causing excessive specific energy requirements.
Metallic sodium is commercially produced almost exclusively in a Downs-cell such as the cell described in U.S. Pat. No. 1,501,756. Such cells electrolyze sodium chloride that is dissolved in a molten salt electrolyte to form molten sodium at the cathode and chlorine gas at the anode. The cells operate at a temperature near 600° C., a temperature compatible with the electrolyte used. Unlike the sulfur anode, the chlorine anode is utilized commercially both with molten salts as in the co-production of sodium and with saline solution as in the co-production of sodium hydroxide.
Another cell technology that is capable of reducing electrolyte melting range and operation of the electrolyzer to less than 200° C. has been disclosed by Jacobsen et al. in U.S. Pat. No. 6,787,019 and Thompson et al. in U.S. Pat. No. 6,368,486. In those disclosures, low temperature co-electrolyte is utilized with the alkali halide to form a low temperature melting electrolyte.
Gordon in U.S. Pat. No. 8,088,270 teaches the utilization of solvents which dissolve sulfur at a cell operating temperature and dissolving sodium polysulfide in such solvents to form an anolyte which when introduced into a cell with an alkali ion conductive membrane are electrolyzed to form sulfur at the anode and alkali metal at the cathode and where a portion of the anolyte is removed from the cell, allowed to cool until the sulfur precipitates out.
It is an object of the present invention to provide a cost-effective and efficient method for the regeneration of alkali metals used in the desulfurization, denitrogenation, and demetallation of hydrocarbon streams. As will be described herein, the present invention is able to remove contaminants and separate out unwanted material products from desulfurization/denitrogenation/demetallation reactions, and then recover those materials for later use.
Another objective of the present invention is to teach improvements in the process and device for recovering alkali metal from alkali metal sulfide generated by the sulfur removal and upgrading process.